A highly utilitarian battery powered two wheel hand truck has been disclosed and claimed in U.S. Pat. No. 3,907,138. A companion patent, U.S. Pat. No. 3,901,396 claims the method of using certain features of the disclosed battery powered two wheel hand truck in moving loads up and down the stairs by a walking technique. The disclosures in both U.S. patents are essentially the same, but the older U.S. patent claims the apparatus in which an elongated screw shaft is cooperatively and advantageously in constant contact with a brake which force is insufficient to prevent the lead screw from operating when the electric motor is turned on. Yet, the force of the brake is sufficient to hold the load carried by the two wheel hand truck when the electric motor is turned off. This instant and automatic braking action operates simply by turning the motor off, and yet the braking action is instantly overcome by the torque of the lead screw when the motor is turned on. Such instant engagement and release of the braking action allows a truck of this type to be successfully used in a method for precise and quick carrying of loads up and down the stairs.
A brief description of the use of the battery powered two wheel hand truck for moving loads up and down the stairs will illustrate why such two wheel hand truck is incapable of operating as a stacker. The wheels are attached to the bottom of the stationary frame which never moves, only the inner frame moves, and it never moves above the wheels at the bottom of the stationary frame. To move loads up the stairs, the truck is first at ground contact at the foot of the stairs with the bottom of the inner movable frame aligned generally with the bottom of the stationary frame. A switch is operated to start the power means by operating an electric motor so that the inner movable frame moves downwardly or below the wheels, the only way such frame moves relative the wheels. The inner frame instantly contacts the ground surface and this causes an opposite reaction which raises the wheels in the stationary frame above the inner frame. The inner frame, in a sense, is moving against the immovable floor so the stationary frame, and its attached wheels, have no place to go but up. The operator allows the stationary frame and wheels to rise to a selected stair height quickly and precisely. The operator turns off the switch so that the load carried by the two wheel hand truck is instantly held by the brake which is always operatively engaged to the power means. This allows the operator to tilt the load to clear the bottom stair, whereupon the operator turns on another switch to reverse the power means so that now the inner movable frame moves up towards the wheels resting on the stair. The operator selects a stair where he wants a load to stop at the bottom of the load and quickly turns off the switch. This precisely and instantly stops the movement of the inner load at such selected stair and the brake means instantly hold the load. This stair is usually one or two stairs below the stair on which the wheels are, unless the stair is wide enough to accomodate both the wheels and the load. The process is repeated until the load is walked up the stairs.
The reverse procedure is followed to walk a load down the stairs, that is, first lowering the inner frame below the wheels to engage a selected lower stair, then moving the wheels to clear an upper stair. Another switch is operated to raise the inner movable frame, but such action results in the stationary frame and the wheels being lowered to a selected stair because such wheels are not in ground contact. There is no point of contact except the load on the stairs so "raising" the inner movable frame does not allow the frame to move to any other place, thus resulting in the stationary outer frame and the wheel being lowered selectively in a quick and precise manner to a selected stair. The procedure is repeated until the load is walked down the stairs.
The foregoing illustration shows that the two wheel hand truck, as disclosed in the foregoing patents, is incapable of operating as a stacker, that is, of lifting the load on the movable frame above the wheels at the bottom of the stationary frame.